1. Field of the Invention
The present invention relates to a thin film transistor liquid crystal display (TFT LCD), and more particularly, to a TFT LCD that is improved in its brightness by varying the locations of TFTs.
2. Description of the Related Art
In general, a cathode ray tube (CRT) has been one of the most popular display devices. But it is becoming inconvenient to use it because of its large size and heavy weight characteristics compared with its display area.
Accordingly, a thin flat panel display has been developed, which can be installed and used anywhere owing to its slimness characteristic despite its large display area. The thin flat panel display is being substituted for the CRT. In this regard, a liquid crystal display (LCD) has a more excellent resolution than other display devices and a response speed as fast as the CRT in displaying a moving picture.
As is well known to those skilled to the art, the operation principle of the LCD is based on the optical anisotropy and polarization property of liquid crystal molecules. Since each liquid crystal molecule has a thin and long structure, it is possible to control the alignment of the liquid crystal molecules having certain orientation and polarization properties by artificially applying an electromagnetic field.
By properly adjusting the orientations of the liquid crystal molecules, it becomes possible for the liquid crystal molecules to allow light to transmit or to be shielded by using their optical anisotropic characteristic, thereby realizing the colors and image.
Generally, in the liquid crystal display, a first substrate (or a thin film transistor substrate) and a second substrate (or a color filter substrate) are provided to face each other with a predetermined interval therebetween.
Describing in more detail, formed on an inner surface of the TFT array substrate are a plurality of gate lines and a plurality of data lines that are arranged crossing perpendicularly with each other to define a matrix configuration.
Formed on respective crossing points of the gate lines and the data lines are TFTs that function as switching elements. Square pixel electrodes coupled to respective drain electrodes of the TFTs are formed on respective regions defined by gate bus lines and data bus lines. Formed on an inner surface of the color filter substrate facing the TFT array substrate are a black matrix (BM) layer, color filters, and a common electrode.
When a voltage is applied to the gate and data lines, the TFTs formed on the crossing points of the gate and data lines to which the voltage is applied are turned on to accumulate electric charges on the pixel electrodes connected to the drain electrodes of the TFTs that are turned on, thereby varying the orientation of corresponding liquid crystal molecules between the pixel electrodes on which the electric charges are accumulated and the common electrode.
FIG. 1 is a perspective view of a related art TFT LCD.
As shown in FIG. 1, the related art TFT LCD includes an upper substrate 105 with a black matrix layer 106 and a color filter layer 108, and a transparent common electrode 118 formed on the black matrix layer 106 and the color filter layer 108; and a lower substrate 122 having pixel regions P on which pixel electrodes 117 are formed and on which a TFT array including a plurality of switching elements T (TFTs) is formed. A liquid crystal layer is formed in a space defined between the upper and lower substrates 105 and 122.
The upper substrate 122 is a TFT array substrate on which TFTs are arranged in a matrix configuration. The TFTs are disposed on respective crossing regions where gate lines 113 intersect data lines 115. The pixel regions P are defined by the intersection of the gate lines 113 and the data lines 115. The pixel electrodes 117 formed on the respective pixel regions P are formed of a conductive metal such as indium-tin-oxide (ITO) having high light transmissivity.
The orientation of the liquid crystal layer on the pixel electrodes 117 is determined by a direction of electric field applied from the TFTs. The light transmission through the liquid crystal layer is adjusted in accordance with the orientation extent of the liquid crystal layer to display a desired image.
FIG. 2 shows an enlarged view of a TFT array substrate of the related art TFT LCD in FIG. 1, and FIG. 3 shows a sectional view taken along the line A–A′ of FIG. 2.
As shown in FIGS. 2 and 3, formed on the TFT array substrate 122 is a transparent insulating layer 110 on which the gate lines 113 and gate electrodes 130 extending from the respective gate lines 113 are formed. The gate lines 113 and the gate electrodes 130 are formed of a conductive material such as metal.
A gate insulating layer 142 is deposited on the transparent insulating layer 110 including the gate lines 113 and the gate electrodes 130. On the gate insulating layer 142, an active layer 136 and an ohmic contact layer 138 are sequentially formed.
Formed on the ohmic contact layer 138 are the data lines 115 perpendicularly crossing the gate lines 113, source electrodes 132 extending from the data lines 115, drain electrodes facing the respective source electrode 132 based on the respective gate electrodes 130, and capacity electrodes C overlapped with the drain and gate lines 134 and 113.
In addition, all of the data lines 115, the source and drain electrodes 132 and 134 and the capacity electrodes C are covered with a passivation layer 170. The passivation layer 170 is provided with contact holes 171 and 140 through which the drain electrodes 134 and the capacity electrodes C are exposed, respectively.
The pixel electrodes 117 are formed on portions of the passivation layer 170, which correspond to the pixel regions defined by the intersections of the gate lines 113 and the data lines 115. The pixel electrodes 117 are connected to the drain electrodes 134 and the capacity electrodes C through the contact holes 171.
Describing the operation of the above-described TFT array substrate, when a voltage is applied to the gate electrodes 130 through the gate lines 113, electrons are collected on the active layer 136 to define a conductive channel, thereby allowing current to flow between the sources and drain electrodes 132 and 134. At this point, image signals transmitted to the data lines 115 reach the pixel electrodes 117 through the source and drain electrodes 132 and 134.
The above-described liquid crystal display uses a backlight as a light source, which is disposed on a rear side of the TFT array substrate. The light radiated from the backlight is attenuated while passing through the LCD. That is, only 3–8% of the incident light passes through the LCD, deteriorating the brightness of the screen.
To solve this problem, a method for improving the brightness of the screen by increasing the brightness of the backlight has been proposed. However, this method is not preferable because it increases the electric power consumption for the LCD significantly.
Therefore, in order to improve the brightness of the screen without increasing the electric power consumption, a method for enlarging an apparatus ratio of a liquid crystal panel by increasing an area occupied by the pixel regions has been also proposed. To increase the area of the pixel regions, there is a need to reduce a width of the data line and a gap between the pixel electrodes.
However, when the gap between the pixel electrodes is reduced, the distance between the pixel electrode and the data line is also reduced. As a result, a coupling phenomenon is incurred by electric attraction between them, thereby deteriorating the drive of the TFTs.
A method for improving the brightness of the LCD by using an optical film has also been developed. However, an additional process for forming the optical film is further required which increases the manufacturing cost of the LCD.